More OOP: Exercise 1 - Modify Scoreboard:

Encapsulate the data members of the Scoreboard class.
Add appropriate accessor and mutator methods.

- homeTeamName can't be an empty string or a null object.
- visitingTeamName can't be an empty string or a null object.
- If either homeTeamName or visitingTeamName is given an 
  invalid string, use "Home"/"Visitors" instead.

- homeTeamScore and visitingTeamScore can't receive a negative
  value, otherwise an exception is thrown.